The present invention relates to a delayed packet concealment method and apparatus in a packet network that uses delayed parameters to improve concealment of delayed packets.
Digitally compressed speech signals are often transmitted in packets containing speech parameters for reconstructing speech frames in a decoder at the receiving end. Typical examples of such packet networks are IP and ATM networks. When packets are delayed or lost, some sort of concealment method is used to cover for the delayed or lost speech parameters (see citation ([1]). Typically these concealment methods comprise predicting the speech parameters for a delayed or lost packet from previously received parameters, and applying the predicted parameters to the decoding process instead of the delayed or lost parameters. The parameters of the first delayed or lost packet are usually simply copied from the previous packet. If further packets are delayed or lost, the same parameters are still used, but now the output signal is gradually muted. A characteristic feature of these methods is that the same strategy is used both for delayed and lost packets. A drawback of these methods is that the information in delayed packets is simply discarded, although it is more up to date than the information that is used for parameter prediction.
A method that distinguishes between delayed and lost packets is described in citation [2]. In the method described in this document speech parameters in delayed packets replace predicted parameters as soon as the delayed packet arrives. However, a characteristic feature of this method is that it does not consider the fact that the decoder is based on digital filtering. Digital filters in the decoder reach final filter states after decoding of a frame. These final filter states are used as initial filter states for the decoding of the next frame (with the new speech parameters). If the decoded output signal is to be the same signal as the optimal signal that was produced in the analysis-by-synthesis process in the encoder at the transmitting end, both speech parameters and initial filter states have to be the same. In the method described in citation [2], only the correct speech parameters will be used when a delayed packet eventually arrives. However, in the meantime the filter states have drifted away from the final state of the previous frame during the prediction phase, which leads to annoying abrupt output signal changes when the delayed speech parameters are suddenly applied.
An object of the present invention is to provide a delayed packet concealment method and apparatus that uses information in delayed packets, but in which such annoying abrupt output signal changes are minimized or even eliminated.
Briefly, the present invention involves using the information received in delayed packets to update not only the speech parameters, but also the initial decoder state. During delayed packet concealment two decoded output signals are then generated with the same speech parameters, one based on drifted decoder states and one based on updated decoder states. Thereafter these two output signals are weighted together into a final output signal. This procedure makes the transition from predicted to updated speech parameters smoother.